1. Field of the Invention
The present invention relates to a separator for a fuel cell which is mainly used as a cell for an electric vehicle, and more particularly to a separator for a fuel cell of the electrolyte type in which a unit cell that is a unit constituting the cell is configured by: sandwiching a gas diffusion electrode having a sandwich structure wherein an electrolyte membrane configured by an ion exchange membrane is interposed between electrodes (an anode and a cathode); and forming fuel gas passages for a fuel gas containing hydrogen and oxidant gas passages for an oxidant gas containing oxygen between the separators, and the electrodes of both the sides.
2. Description of the Prior Art
In a fuel cell, a fuel gas and an oxidant gas are respectively supplied to an anode and a cathode, so that, in the anode and the cathode, electrochemical reactions indicated by the formulae:
H2xe2x86x922Hxe2x88x92+2exe2x88x92xe2x80x83xe2x80x83(1)
(xc2xd)O2+2Hxe2x88x92+2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
occur, and, in the whole of the cell, an electrochemical reaction indicated by the formula:
H2+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
proceeds. The chemical energy due to the chemical reaction is converted into electrical energy, with the result that the cell can expect a predetermined performance.
A separator for a fuel cell of the electrolyte type in which such energy conversion is conducted is desirably gas-impermeable, and also is made of a material of high electrical conductivity in order to improve energy conversion efficiency. Conventionally, as a material meeting the requirements, a complex which is configured by mixing graphite (carbon) powder with a thermosetting resin such as phenol resin, or a so-called bondcarbon (resin-bonded carbon) compound is used, and a separator for a fuel cell is configured by forming such a bondcarbon compound into a predetermined shape.
Conventionally, a separator for a fuel cell having a predetermined shape is formed by using such a bondcarbon compound in the following manner. With respect to the composition ratio of a thermosetting resin such as phenol resin and graphite powder, a bondcarbon compound in which the content of the thermosetting resin is increased is used placing emphasis on moldability. In the case of injection molding which is molding means having good productivity, for example, a bondcarbon compound in which the content of a thermosetting. resin is set to about 40% or more is used.
In a conventional separator for a fuel cell molded by using a bondcarbon compound in which the composition ratio of a resin is large as described above, moldability is excellent because the bondcarbon compound is excellent in fluidity, but the content of a thermosetting resin serving as an electrically insulating material is large, and hence the specific resistance of the bondcarbon compound itself constituting the separator is raised. In the case where injection molding having good productivity is employed as molding means, particularly, the composition ratio of a thermosetting resin must be set to about 40% or more, and therefore the specific resistance of the bondcarbon compound is largely raised to 1xc3x9710xe2x88x921 to 1 xcexa9xc2x7cm. The specific resistance is one of performances of materials constituting a separator. The resistance of a separator is largely affected also by other factors, i.e., the internal resistance and the contact resistance. Among such factors, the contact resistance with respect to an electrode exerts the largest effect. In a separator of the conventional art in which no countermeasure against the above-mentioned phenomenon is taken, therefore, the resistance remains to the large specific resistance of a bondcarbon compound having a large composition ratio of a resin, so that the electrical resistance of a portion which is to be contacted with an electrode is high. This is not preferable from the viewpoint of the performance of a fuel cell.
In order to lower the specific resistance of a bondcarbon compound itself serving as a material constituting a separator so as to improve the conductivity of a separator for a fuel cell, it may be contemplated that the content of a thermosetting resin is reduced. When a bondcarbon compound in which the content of a thermosetting resin is reduced as described above is used, however, elongation and fluidity of the bondcarbon compound during a molding process are lowered to impair the moldability, and easily cause molding unevenness. As a result, it is impossible to obtain a molded member (separator) which is correct in shape. Furthermore, height uniformity and flatness of an end face contacting with an electrode are largely affected by the accuracy of a molding die. Even when the specific resistance of the bondcarbon compound itself is lowered, therefore, the contact resistance with respect to an electrode is high, and hence it is difficult to sufficiently improve the conductivity of a separator.
Because of the above-discussed reasons, in a separator for a fuel cell of the conventional art, a bondcarbon compound in which the composition ratio of a resin is large is usually employed as described above with placing the primary emphasis on moldability. As a result, the specific resistance of a bondcarbon compound itself is high, and the contact resistance with respect to an electrode which largely affects the performance of a fuel cell is inevitably raised, so that the problem of reduced conductivity is unavoidably produced.
The present invention has been conducted in view of the above-mentioned situations. It is an object of the invention to provide a separator for a fuel cell in which, while excellent moldability is ensured by using a bondcarbon compound of a large resin content, the contact resistance with respect to an electrode can be lowered so that the conductivity of the whole can be improved.
It is another object of the invention to enhance flatness of an end face contacting with an electrode without being affected by the molding accuracy of the whole of a separator, whereby the contact resistance with respect to an electrode can be sufficiently lowered.
It is a further object of the invention to lower the contact resistance with respect to an electrode and, at the same time, enhance the bending elasticity of a separator, so that also damage, such as crack due to vibrations, can be prevented from occurring.
In order to attain the objects, the separator for a fuel cell of the invention is a separator for a fuel cell of an electrolyte type consisting of a complex which is configured by mixing graphite powder with a thermosetting resin, and characterized in that, in the complex, a composition ratio of the graphite powder is set to 60 to 90%, and a composition ratio of the thermosetting resin is set to 10 to 40%, and at least an end face, contacting an electrode, is coated with a conductive film made of a material which is lower in specific resistance than the complex.
According to the invention having the above-mentioned characteristic configuration, a bondcarbon compound of a complex in which the composition ratio of the thermosetting resin is set to be large or 10 to 40% is used, and hence elongation and fluidity during a molding process are improved so that, even when injection molding means having good productivity is employed, a molded member (separator) of a desired shape can be surely obtained. While ensuring such excellent moldability, the contact resistance between the molded member and an electrode which is affected at the highest degree by the increased content of the resin can be greatly lowered by the presence of the conductive film with which the end face contacting with the electrode is coated. Therefore, it is possible to attain an effect that, while improving the productivity by the employment of injection molding means having excellent moldability, the conductivity of the separator as a whole can be remarkably improved and predetermined performance of a fuel cell can be sufficiently exerted.
In the separator for a fuel cell of the invention, the conductive film may be thin or have a thickness of 10xcexcm or smaller, preferably, 3xcexcm or smaller. According to this configuration, the flatness accuracy of the conductive film can be enhanced without being affected by the molding accuracy of the separator itself, so that the actual contact area can be made larger. This cooperates with the low specific resistance of the film itself to reduce the contact resistance with respect to an electrode to an extremely low level.
As a material of the conductive film in the separator for a fuel cell of the invention, any conductive material such as conductive graphite paste, gold paste, or silver paste may be used as far as the conductive material is lower in specific resistance than the bondcarbon compound.
As means for performing coating of the conductive film, any means such as spraying, vapor deposition, printing, or application may be employed as far as a smooth surface is attained and the means exerts excellent adaptability with an electrode.
As the thermosetting resin which is useful in the invention, phenol resin which is excellent in wettability with respect to graphite powder may be most preferably used. Alternatively, any other resin such as polycarbodiimide resin, epoxy resin, furfuryl alcohol resin, urea resin, melamine resin, unsaturated polyester resin, or alkyd resin may be used
As the graphite powder which is useful in the invention, powder of graphite of any kind, including natural graphite, artificial graphite, carbon black, kish graphite, and expanded graphite may be used. In consideration of conditions such as cost, the kind of graphite can be arbitrarily selected. In the case where expanded graphite is used, particularly, a layer structure is formed by expanding the volume of the graphite as a result of heating. When molding pressure is applied, layers can twine together to be firmly bonded to one another. Therefore, a complex and hence a separator can be enhanced in bending elasticity so that damage, such as cracks due to vibrations, can be prevented from occurring. Consequently, expanded graphite is most effective.
Other objects and effects of the invention will be clarified in embodiments which will be description below.